The present invention relates to activation sensing and more particularly to activation sensing to determine activation of a bleed valve particularly in high pressure or intermediate pressure compressor stages of a gas turbine engine.
Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts.
With regard to gas turbine engines control is important in terms of operation of the engine as well as confirmation that a particular control strategy has been performed. In such circumstances the provision of intermediate and high pressure bleed valves in the turbine and compressor stages of an engine is used to control stability through the engine and therefore performance. Intermediate and high pressure bleed valves do not generally include their own direct position feedback control or other sensing of activation. This is due to the inherently arduous environment within which those bleed valves operate. It will be understood that if the feedback device indicates failure of the bleed valve to respond when that valve is operated correctly, then there will be more indications of failure than necessary, leading to additional costs with respect to maintenance, downtime and unnecessary repairs. In such circumstances indirect approaches can be taken with regard to confirmation that bleed valve operation and activation has occurred.
An example of a prior indirect approach to determine bleed valve operation includes a specific test of bleed valve operation carried out at least once per flight during descent whilst the engine is idle. Thus, a sensor for compressor pressure is monitored for a period, typically 0.5 seconds, then a bleed valve is opened. The compressor pressure is then monitored for a further period of up to 1.5 seconds or until the required change in the compressor pressure is observed indicating that a valve has opened. If the desired time passes without the required change in the compressor pressure being observed, the valve is deemed not to have opened. If the valve opening test is passed the closing test is performed in a similar way, that is to say waiting for a period of typically 10 seconds until the compressor pressure has stabilised. It will be understood that typically there is more than one bleed valve in an engine such that there may be a three bleed valve arrangement with each valve requiring testing in turn, the total elapsed time including stabilisation periods will be in the order of 90 seconds. However, if during this test period the engine ceases to be idle or if the compressor pressure ceases to be steady for whatever reason, then the test must be aborted and reinitiated at least for the values yet to be tested. It will be understood that similar systems are utilised with respect to testing of bleed valves on the ground.
Disadvantages with regard to the above prior techniques include the necessity of switching the bleed valves simply to test them rather than detecting actual switching during normal operation. In such circumstances the bleed valves are switched more times than necessary and this may shorten their operational life. It will also be understood that being dependent upon the normal compressor pressure value being steady other effects not due to bleed valve switching can render the results unreliable and, as indicated, can cause problems with respect to the necessity of having 90 seconds of stability or whatever period to test all bleed valves in this steady state condition. Thus, if there is any thrust or throttle movement with respect to deviating the engine power over from an idle condition it will be understood that the bleed valve test must be aborted. Unfortunately, such thrust changes are commonplace such that there is anecdotal evidence that bleed valve testing is seldom completed at least for all bleed valves.